This invention relates to an aircraft warning and/or control system and more particularly to a system for warning a pilot or activating an automatic throttle system to decelerate an aircraft as it approaches a preselected altitude.
In the United States, the Federal Aviation Administration (FAA) requires that an aircraft""s speed be no greater than 250 knots indicated airspeed at altitudes of less than 10,000 feet (MSL). For this reason, a pilot is generally required to decelerate the aircraft as the aircraft descends from a cruise altitude through 10,000 feet and at other times to decelerate an aircraft as it descends in view of the aircraft""s structural limitations.
In the prior art, such decelerations were generally accomplished by increasing the pitch attitude of the aircraft in a manner proportional to the difference between the new speed command and the actual speed of the aircraft or speed error. This approach will decelerate the aircraft, however there is no assurance that any altitude restrictions will be met. In other prior art systems, the deceleration of the aircraft in the descent phase is accomplished by the computation of a flight path angle used to manipulate the actual vertical path of the aircraft for the achievement of a specified touch down point. Such systems suffer from relatively long deceleration times beginning at relatively high altitudes above the desired point where the commanded speed is to be achieved. Therefore, the deceleration is not optimal or timely, particularly in the aircraft terminal area where precise and rapid speed control changes are important.
A more recent approach for the automatic deceleration in aircraft during descent is disclosed in the U.S. patent of Greeson et al., U.S. Pat. No. 4,633,404. As disclosed therein a deceleration control apparatus for an aircraft has an automatic altitude capture and hold system and an airspeed hold system, both systems controlled by controlling pitch attitude. During descent from a higher altitude under airspeed-on-pitch control with throttle set at idle thrust, and at some existing negative altitude rate, a slower speed is commanded. A synthetic altitude based on the existing descent rate is computed and the altitude capture and hold system is switched into control in place of the airspeed or pitch control to cause the aircraft to flare toward the synthetic altitude. The resultant loss of airspeed during the altitude capture flare is monitored and when the commanded airspeed reduction is achieved, the airspeed-on-pitch control is resumed.
A further approach to descent guidance by a flight management system is disclosed in the U.S. patent of Liden, U.S. Pat. No. 5,739,770. The Liden system determines the current altitude and a corresponding current horizontal distance of the aircraft from a selected bottom of descent point. A simulation of a basic descent path is performed and a plurality of corresponding altitude and horizontal distance points is stored in a basic descent path array. A horizontal distance is determined from the bottom of descent point array which corresponds to the aircraft altitude. An aircraft symbol is displayed on the navigation display unit to indicate the distance remaining until start of descent.
Notwithstanding the above, it is presently believed that there is a need and a commercial demand for an aircraft warning and control system which is responsive to altitude and vertical speed. It is believed that there is a commercial demand for such systems which will ensure a pilot that the aircraft will descend through a preselected altitude without exceeding a preselected airspeed.
In addition, it is presently believed that the warning and control system in accordance with the present invention can be manufactured and incorporated in an aircraft at a competitive cost, will provide accurate and reliable control of the aircraft and/or warning to a pilot and can be incorporated in a conventional automatic throttle system.
In essence, an aircraft warning and/or control system in accordance with the present invention reduces the airspeed of an aircraft as it passes through a preselected altitude or warns a pilot when it is time to do so. The system includes a computer and means for inputting a first preselected altitude and a preselected speed into the computer. The system also includes means such as a barometric altimeter for determining the current altitude of the aircraft and means for measuring the rate of descent or vertical velocity of the aircraft. Means responsive to the current altitude and rate of descent is provided for generating a signal indicative of the time to reach the preselected altitude and/or to reduce the power of the aircraft to reach the preselected speed at the preselected altitude.
In other words, the system identifies a variable point at which to reduce power in order to reach a predetermined air speed at a preselected altitude. This variable point is a function of vertical speed, the air speed at which the aircraft is flying and the altitude left to reduce the air speed to the preselected airspeed at the preselected altitude. Assuming that the airspeed of the aircraft is to be reduced to 250 knots at 10,000 feet, there will be a different variable point for an aircraft flying at 300 knots and one flying at 400 knots. Similarly, there will be a different variable point for an aircraft that is descending at a rate of 2,000 feet per minute and one that is descending at 1,000 feet per minute.
The invention will now be described in connection with the accompanying drawings.